In the art of oil exploration, a tree is a pressure safety device consisting of a tee-piece on a wellhead which allows vertical intervention and allows fluids to flow through a horizontal or angled lateral port into or out of the well bore. The flow junction in either a surface or subsea tree is achieved using a tree block. The junction is machined from a solid block because a welded junction would not meet the necessary integrity and bending moment requirements. The junction is generally at a 90° angle.
The parent metal of the body, usually steel, is rarely suitable to handle the range of fluids found and used in a production oil well. The possibility of corrosion, erosion, hydrogen or carbon dioxide embrittlement requires the wetted surfaces of the bores and the outer portions of the body itself to be protected. This protection is usually achieved by nickel alloy such as Inconel® lining the surfaces by continuous welding of a wall of nickel alloy material to the parent metal wall. When striking an arc, and until the weld is established, an inconsistent quality weld bead is laid which is not acceptable. The same occurs when the weld is stopped and started, which occurs when the weld gun has to traverse an opening or side port. To clad steel surfaces in nickel alloy is a complex welding procedure. To clad the surfaces of the bore using nickel alloy welding, an electric welding arc must be kept at a precise distance from the metal surface to ensure a consistent weld quality. At a junction on a tree block between a production bore and a side outlet, the rotating weld has to jump the bore and then resume once across. The quality when the weld arc is resumed can suffer. A known solution to this problem is shown in FIGS. 1a to c. 
FIG. 1 shows a typical wellhead tubing hanger with its lower end on the right (the wellhead is not shown in FIG. 1) at various stages in its manufacture. FIG. 1a shows a cross section of a tubing hanger which has a substantially circular cylindrical body 10. A bore 11 is machined in the body 10 which in use will provide a passage for production fluid or water injection fluid. An opening 12 is machined on the side wall of the body 10. In use, either production fluid will flow from the bore 11 to the opening 12 or water or gas injection flow fluid will flow from the opening 12 to the bore 11. The flow of fluid in FIG. 1 is from the right hand side of the bore 11 to the opening 12, or vice versa.
As described above, it is necessary to coat all well fluid wetted surfaces of the tubing hanger with nickel alloy. The surfaces that need to be clad are the inner surface 13 of the bore 11 and certain portions of the outer diameter 14 of the tubing hanger. To ensure a consistent well quality, a solid cylindrical nickel alloy plug 15 of circular cross section is inserted into the opening 12 as shown in FIG. 1b. This results in the surface of the bore 11 and the outer diameter 14 of the body 10 having a substantially flush surface, all the way around the body 10. In the next step, an electric arc is used to nickel alloy clad the body 10 to create cladding 16 on the surface of the bore 13 and cladding 17 on the outer diameter 14 of the body 10. The nickel alloy plug 15 allows the electric arc to be kept at a constant distance from the outer diameter 14 of the body 10 when the arc rotates around the outside of the body, or the body is rotated around the arc.
The next step of the process is shown in FIG. 1c where the center of the nickel alloy plug 15 is machined out leaving a sufficient amount of nickel alloy on the side walls of the opening 12. As can be seen, all the well fluid wetted surfaces of the tubing hanger are now clad in nickel alloy. Although the process shown in FIGS. 1a to 1c helps to provide a consistent weld quality, the use of a sacrificial nickel alloy plug is undesirable because it requires considerable machining time to bore out the plug. Alternatively, instead of using an insert plug, the opening 12 is welded solid using nickel alloy.
For pressure containing equipment, especially oil field hydrocarbon pressure containing equipment, the metallurgic consistency structure of the parent body to withstand the hydrostatic forces is paramount. The body must have an equal consistency throughout its shape. Precise geometric surfaces, which are required in this field, cannot be achieved by casting and hence the bores are machined in the solid forged body.